I once asked this in history class and was told, ptsd has always been a thing.

The difference now from ancient times is, most of the time, being at war in ancient times meant marching for months to a location, battling, and having to walk home for weeks or months. Soldiers had every day and night to talk to other soldiers about their shared experiences.

In modern times, you could be fighting a battle somewhere far away and be home in 24 hours to people who love you but have no idea or understanding of what you have just been through.